In order to detect the concentration of methane in water， a sensor system of dissolved methane in water based on off-axis integrated cavity output spectroscopy is developed in this paper. A distributed feedback laser （center wavelength， 1 653 nm）， a laser temperature control module， a laser current drive module， a resonator cavity/gas chamber， a photoelectric detector， a data acquisition module， a data processing module and a gas-liquid separation module were included in the system. Experiments including calibration of effective optical path length， calibration of direct absorption signal， and stability test were carried out by using methane gas samples and pure nitrogen （N₂）. The effective optical path length of the cavity is calibrated using a methane sample with a concentration level of 10×10^-6， which is determined to be 1 906 m. Using pure nitrogen as the target gas， the stability of the system is measured. The results of Allan variance analysis show that when the averaging time is 2 s， the detection sensitivity is 92.8×10^-9， and the sensitivity of the system can be improved to 13.2×10^-9 with an averaging time of 134 s. The system was used to measure the concentration of the dissolved methane in tap water， rainwater and lake water， and the results confirmed the engineering practical value of the technique and system. The research work and related results have laid a good foundation for water quality detection and exploration of clean energy such as natural gas hydrate.